Many portable electronic devices such as handheld wireless communications devices (mobile phones or smart phones) and notebook or laptop personal computers have built-in, switching power supply circuits that are powered by an external power source. For instance, when an AC power adapter having a regulated output is plugged into a smart phone, a switching power supply circuit in the smart phone draws current from the plugged-in power adapter, and produces the needed voltage for charging the device's battery and/or for operating the rest of the components of the device. The way in which the switching power supply circuit draws current, however, creates a negative impedance load. As a result, the external power source sometimes behaves abnormally, e.g. its normally stable dc output drops out of regulation, exhibits excessive undershoot and/or overshoot, and may even oscillate.